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The Doctor and Ian visit Prague in 2908 to find the streets mostly deserted. As they walk the streets, the Doctor tells Ian about the three political factions in Prague. They soon learn that there is a plague, named the "Blue Plague", that has put many patients in a coma. As the Doctor does not remember a plague in Prague's history, they go to a hospital to investigate.

At the hospital, the Doctor looks at a patient and takes a sample of his tears. He uses a computer to investigate the virus, convincing a security guard that he has permission. Meanwhile, Ian eavesdrops on a conversation between two doctors, who eventually come to blows. While the guard investigates, the Doctor and Ian head to the lab to study the tear sample.

The Doctor tells Ian that the virus was created by one of the political factions, the Technological Front, using nanotechnology. The Doctor, inspired by a chance comment from Ian, figures out the antidote, giving it first to Ian, who has by now caught the virus. He then inoculates several patients, attracting the attention and thanks of the hospital administrator, Dr Korshinski.

When the Doctor reveals to the staff the source of the infection, an argument starts between most of the staff and a doctor from the Technological Front who was unaware of the source. While they sort out their dispute, the Doctor and Ian make a discreet exit.